Glasses are very useful dielectric materials because they can be easily formed into a wide variety of shapes and also because they can be chemically bonded to metals to form a hermetic seal. However, they are brittle materials with low toughness, this being a limit to their use in structural applications. The toughness of glasses may be improved by dispersing in them particulates, such as whiskers and fibers. In most cases however, the improved toughness is accompanied by reduced strength because these additives increase the size of the inherent internal flaws that ultimately cause failure.
Glass ceramics exhibit high strength while still retaining the ability of being easily formed into complex shapes by means of standard glass forming techniques, as well as the ability to form hermetic seals. Glass ceramics are produced by melting a glass-forming batch, cooling the melt and shaping a glass article therefrom, and subsequently heat-treating the glass article within a particular temperature range for the period of time necessary to develop the desired internal crystallization. This crystallization or precipitation process permits precise control of crystal size, volume fraction, and distribution in the final material.
Conventional ceramic processing requires the mechanical mixing of the component crystalline phases, followed by heating. This process results in an inherent lack of homogeneity, along with a strict dependence of the final crystalline microstructure upon the starting materials.
It has been disclosed in the prior art that both the strength and toughness of a conventional ceramic material can be improved by dispersing particles of ZrO.sub.2 in the original mixture of the material. This has been done with alumina ceramics, as reported in "Design of Transformation-Toughened Ceramics" by N. Claussen and M. Ruhle in Science and Technology of Zirconia, A. Heuer and L. W. Hobbs, eds, American Ceramic Society, 1981. It is postulated that the ZrO.sub.2 particles improve the material toughness in the following manner. Assuming that the ZrO.sub.2 is retained in the metastable tetragonal phase, the ZrO.sub.2 particles are transformed into the stable, monoclinic phase in the stress field near a crack tip and shield the crack tip from an applied stress. Because the crystals can be kept small and still can be transformed, the inherent flaws that they introduce are also small, and the strength of the material is thereby increased. It should be noted that these ZrO.sub.2 particles are initially added to the original starting materials and remain as crystalline ZrO.sub.2 throughout the entire processing sequence.
The use of zirconia in glass ceramic bodies has also been disclosed. For example, in U.S. Pat. No. 3,252,811, zirconia is incorporated as a nucleating agent to produce a transparent glass ceramic body with very high strength and excellent resistance to thermal shock. The use of TiO.sub.2 and ZrO.sub.2 as nucleating agents in glass ceramic articles is taught in U.S. Pat. Nos. 3,926,660 (Andress et al) and 4,126,477 (Reed). A glass ceramic that is particularly adapted for incorporation of radioactive waste is shown in U.S. Pat. No. 4,314,909 (Beall et al). The process for making the glass ceramic employs cubic or tetragonal ZrO.sub.2 solid solution.
Incorporation of tetragonal ZrO.sub.2 in a glass matrix has also been disclosed. For example, in an article entitled "A Structural Study of Metastable Tetragonal Zirconia in an Al.sub.2 O.sub.3 -ZrO.sub.2 -SiO.sub.2 -Na.sub.2 O Glass Ceramic System," by G. Fagherazzi, G. Enzo, V. Gottardi, and G. Scarinci in Journal of Material Science, 1980, pages 2693 to 2700, the structural and microstructural properties of metastable zirconia in a glassy system are discussed. The peculiarly small size of the precipitated zirconia crystallites is confirmed in the stabilization of the tetragonal form of ZrO.sub.2 with respect to the stable monoclinic one, and is explained in terms of a contribution to the amount of free energy due to strain energy in addition to the previously hypothesized surface energy. Another article discussing ZrO.sub.2 in a glass system is "Phase Equilibria in Ternary Systems Containing Zirconia Silica" by A. Sircar and N. Brett, appearing in Ceramic Society volume 69, pages 131-135, 1970. The presence of uncrystallized ZrO.sub.2, remaining as a chemical component of the glass matrix, has also been shown to increase the resistance to attack by aqueous alkali. This is discussed in an article entitled "Chemical Durability of Sodium Silicate Glasses containing Al.sub.2 O.sub.3 and ZrO.sub.2 " by C. R. Das appearing in Journal of the American Ceramics Society, volume 64, No. 4, pages 188-193, April 1981.
It is an object of this invention to provide a glass ceramic toughened with tetragonal zirconia. Another object is to provide a toughened glass ceramic that can be shaped by standard glass-forming techniques. A further object is to provide a glass ceramic that is particularly adapted for use as a structural ceramic insulating material in devices such as neutron tubes and switches. A further object is to provide a glass ceramic particularly suited for forming ceramic fibers for use in composite materials. A still further object is to provide a glass ceramic particularly effective in reinforcing concrete.
Upon further study of the specification and appended claims, further objects and advantages of the invention will become apparent to those skilled in the art.